Combine sugar, syrup, and water in a 2-quart saucepan. Cook over medium heat, stirring constantly, until sugar dissolves. Cover and cook 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, without stirring, until mixture reaches thread stage (230°). Add pecans; continue to cook, stirring constantly, until mixture reaches hard crack stage (300°).
Remove from heat; stir in butter and soda, mixing well. Working rapidly, spread mixture in a buttered 9-inch square baking pan. Cool; break into pieces.